1. Field of the Invention
The present invention relates generally to a liquid jet recording head for performing recording by discharging a liquid (ink) in a jet and causing droplets to fly toward a recording medium, as well as a liquid jet recording apparatus having such a liquid jet recording head. More specifically, the invention relates to a liquid jet recording head which enables gradation recording as well as a liquid jet recording apparatus having the same.
2. Related Background Art
It is well known that use of liquid discharge openings which are integrated at high density is effective in faithfully recording an original image by means of such a liquid jet recording head. In addition, gradation recording is needed in order to faithfully reproduce the gradient of an original image within a limited range of recording density.
Various proposals have been made with respect to liquid jet recording heads, in particular, liquid jet recording heads of the type that employs an electricity-heat conversion member as energy generating means for generating energy used to discharge a liquid to thereby perform gradation recording. U.S. Pat. No. 4,339,762 discloses a structure in which a temperature gradient occurs in a heating portion. U.S. Pat. No. 4,251,824 and Japanese Patent Laid-open No. 132259/1980 disclose an arrangement in which each liquid path includes at least two heating elements which allow at least two independent signals to be input.
FIG. 1A is a diagrammatic perspective view illustrating a conventional type of liquid jet recording head comprised of a plurality of liquid discharge portions which are disposed in an integrated form, each of the liquid discharge portions including a liquid path, a heating portion, and a liquid discharge opening. FIG. 1B is a diagrammatic cross section taken along the line X--X' of FIG. 1A. FIG. 2 is a diagrammatic view illustrating on an enlarged scale the heating portion of a liquid jet recording head which relies upon the related art disclosed in the aforementioned U.S. Pat. No. 4,339,762.
In FIGS. 1, 2 and 3, heating elements 107 (107-1 to 107-6), a common electrode 106, and selectively operable electrodes 105 are disposed over a substrate 103, the electrodes 106 and 105 being provided for energization. The substrate 103 and a grooved cover plate 102 are bonded by adhesive layers 104 (104-1 to 104-7) so that each of the heating elements 107 corresponds to each groove 101 (101-1 to 101-6) which is defined between adjacent ones of separation walls 101a to 101g formed on one side of the grooved cover plate 102. A liquid or ink is introduced into the liquid jet recording head having the above-described arrangement and, when the heating elements 107 are heated, the ink charged into the liquid paths is subjected to rapid pressure changes for causing the ink to bubble. In consequence, the ink is discharged in a jet through the liquid discharge openings which are defined by the grooved cover plate 102 and the substrate 103.
In this related art example, as shown in FIG. 2, the heating elements 107 are constituted by heating resistive elements 108 which are flat in cross section and trapezoidal in plan view. As illustrated, a narrow portion A connected to the common electrode 106 exhibits a high resistance per unit length and a wide portion B connected to the electrode 105 exhibits a low resistance per unit length. Accordingly, the level of the voltage across the heating resistive element 108 which voltage is required to cause generation of a bubble at a portion A is lower than the level of the voltage across the heating resistive element 108 which voltage is required to cause a bubble to be generated at the whole of the resistive element 108. Therefore, the size (or diameter) of a bubble can be controlled through adjustment of the level of drive voltage applied to the heating resistive element 108, thereby enabling gradation recording.
However, in a case where the drive voltage is adjusted so as to cause droplets having the same diameter to be discharged through the liquid discharge openings of such a liquid jet recording head, such droplets having the same diameter may be observed immediately after the ink has been discharged. However, after the ink droplets have been stuck to a recording surface, when the size (or diameter) of each ink dot is measured, there is a case where each dot has a different size.
It has heretofore been considered that the reason for this phenomenon is that, when a bubble is generated at a portion of the heating resistive element 108 (for example, a portion thereof equivalent to 1/3, 1/2 or 2/3 of its whole area which extends from the portion A), a multiplicity of minute bubbles are generated in the vicinity of the boundary between an area in which the bubble is generated and an area in which no bubble is generated, and the minute bubbles cause variations in the speed of droplet discharge, that is, the speed of droplets at the moment they strike the recording surface. Accordingly, the above-described liquid jet recording head has had several problems to be solved in order to achieve stable gradation recording.
FIG. 3 is a diagrammatic view illustrating on an enlarged scale one of the heating portions of a liquid jet recording head which relies upon the related art, such is that disclosed in the aforementioned U.S. Pat. No. 4,251,824 and Japanese Patent Laid-open No. 132259/1980. The illustrated heating portion includes heating elements 301-1 and 301-2, a common electrode 302, and selectively operable electrodes 303-1 and 303-2 which are connected to the heating elements 301-1 and 301-2, respectively. By using this related art heating portion, it is possible to provide step control over the area required for bubble generation by controlling the number of heating resistive elements to be driven, by making the levels of discharge energy of the heating elements 301-1 and 301-2 different from each other to selectively drive one of them, or by providing the phase difference between signals to be independently input to the heating elements 301-1 and 301-2.
In the above-described liquid jet recording head, in order to increase the number of steps of gradation per liquid path, it is necessary to increase the number of heating elements and the number of wires in each liquid path. In this case, however, it becomes difficult to integrate the arrangement of each liquid discharge portion at high density because of limitations imposed by the thicknesses of the wires. This difficulty may hinder faithful reproduction of an original image to a remarkable extent. To the end of avoiding this difficulty, it may be considered that the high-density integration of the liquid discharge portions can be achieved by utilizing multi-layer wiring or the like. However, this proposal involves a complicated process and may cause an increase in the cost of production. In addition, it is necessary for each liquid path to provide drive circuits for inputting at least two independent signals. In some instances, this necessity complicates the structures of liquid jet record heads of the type having a plurality of discharge openings.